High frequency ventilation (HFV) is widely used in ventilation of premature infants because it operates at substantially lower intrapulmonary pressures as compared to conventional positive-pressure ventilation. HFV administers breathing cycles at above about 120 breaths per minute (2 Hz) and as high 1320 breaths per minute (22 Hz) at very low tidal volumes of between about 5 and about 30 ml. Such low volume and high ventilatory rates dictate the use of ventilator circuits having relatively low energy loss and low internal compliance as compared to circuits used with conventional ventilators and ventilation techniques. At the present, ventilator circuits used in administering high frequency ventilation incorporate inspiratory and expiratory tubing limbs of equal tubing diameter, commonly 10 mm inner diameter (ID). The high frequency ventilator circuits include a patient adapter which is substantially symmetrical, having two small diameter pipes each having an outer diameter (OD) of about 10 mm for connecting the tubing. The pipes intersect with a larger diameter pipe for being connected to an endotracheal or tracheotomy tube. Examples of such circuits are neonatal ventilator circuits manufactured and marketed by Hudson Respiratory Care Inc and HFV circuits manufactured and marketed by Purtian Bennett.
In the administration of high frequency ventilation, flow dynamics of inspiratory and expiratory gas flows are particularly critical because of the manner in which the gas is injected into the patient airways at very high flow rates for very brief time intervals. Typically, gas in HFV is delivered at flow rates of up to 50 liters per minute, but because the pressure is administered for only a fraction of a second, only a small amount of gas actually arrives at the distal airways. Moreover, the patient exhales at the same time and against the flow of gas, quite unlike conventional ventilation in which the inspiratory and expiratory cycles occur independently and the inspiratory gas flow is terminated as expiration begins. Accordingly, for HFV, it is desirable to minimize the expiratory back pressure. For this purpose, high frequency ventilators often incorporate an expiratory compensation valve. HFV circuits also use small-diameter tubing having reduced compressible volume and internal compliance, and are of substantially reduced length to further minimize the column of air in the circuit against which or through which the patient must exhale.